1. Field of the Invention
The present invention relates to a video reproducing apparatus, and more particularly, to a video reproducing apparatus where a frequency characteristic of an audio filter circuit is automatically adjusted.
2. Description of the Prior Art
For example, a video cassette recorder (VCR) has a filter circuit in its circuit for processing an audio signal reproduced from a magnetic tape. In realizing such a filter circuit in the form of an integrated circuit (IC), since nonuniformity is generated among each filter circuit in the manufacture process, it is necessary to adjust the characteristics (frequencies) of the filter circuits. Conventionally, this adjustment was made by trimming an internal current or voltage from the outside by manually adjusting a variable resistor externally attached to the IC. According to this adjustment method, the circuit scale of the IC can be small; however, it has drawbacks because the externally attached part and adjustment thereof increases the cost.
Because of the drawbacks of the above method, another method is used in which the adjustment is automatically made by an automatic adjusting circuit provided in the IC. FIG. 1 shows a conventional automatic frequency adjusting circuit for a filter circuit provided in a VCR for separating a reproduced FM audio signal into an left (L) channel component and a right (R) channel component. In FIG. 1, an FM audio signal reproduced from a video track of a magnetic tape 1 by a rotary magnetic head 2 is input to a terminal 3. After the signal has passed through a buffer 4, a luminance signal component is removed from the signal at a low-pass filter 5. Then, the signal is supplied by way of a buffer 6 to band-pass-type first and second filter circuits 7 and 8.
The first and second filter circuits 7 and 8 are provided for separating an L channel signal and an R channel signal from each other. The first filter circuit 7 is formed to have a band whose center frequency is 1.3 MHz. The first filter circuit 7 is constituted by a low-pass filter 7A, a trap filter 7B, a high-pass filter 7C, a trap filter 7D and a band pass filter 7E. The characteristics of the filters 7A to 7E are shown in A to E of FIG. 2, respectively. Likewise, the second filter 8 is constituted by a low-pass filter 8A, a trap filter 8B, a high-pass filter 8C, a trap filter 8D, a band pass filter 8E, which also have characteristics shown in FIG. 2. Outputs of the first and second filter circuits 7 and 8 are directed by way of amplifiers 9 and 10 to output terminals 11 and 12, respectively. To each of the output terminals 11 and 12, an FM demodulator (not shown) is connected.
Since the filter circuits 7 and 8 have high frequencies (1.3 MHz and 1.7 MHz, respectively), even slight nonuniformities or errors generated in the circuit devices constituting the filter circuits 7 and 8 lead to a large frequency difference. For example, a 1% frequency deviation leads to a 13 kHz center frequency deviation in the first filter circuit 7. In the second filter circuit 8, it leads to a 17 kHz deviation. Thus, the difference from the input FM signal is quite large. That is, since in VCRs, for example, with respect to a standard level, the modulation factor for the center frequencies of 1.3 MHz and 1.7 Mhz is 50 kHz, the 13 kHz and 17 kHz frequency deviations of the filter circuits due to the nonuniformities are very large compared to 50 kHz. This causes problems of a hoarse sound and an inaccurate signal level. In order to compensate for the nonuniformities, an arrangement for automatically adjusting a frequency is added. This arrangement is as follows: a 3.58 MHz color subcarrier provided to a terminal 13 is amplified by an amplifier 14, and after an amplitude thereof is limited by a limiter 15, the color subcarrier is provided by way of a low-pass filter 16 and an attenuator 17 to reference filters 18 and 19 and to a phase comparator 20. The phase comparator 20 compares a phase of an output signal of the low-pass filter 16 and a phase of an output signal of the reference filter 19, and provides a comparison output (error signal) by way of a low-pass filter 21 and a controlling circuit 22 to the low-pass filter 16 and to the reference filters 18 and 19 to control so that the error signal becomes 0.
The error signal is also provided from the controlling circuit 22 to the filters 7A to 7E and 8A and 8E constituting the filter circuits 7 and 8 to control frequency characteristics of the filters. In the automatic frequency adjusting circuit of FIG. 1, by providing to a secondary low-pass filter constituted by the reference filters 18 and 19 an arrangement the same as that of the filters 7A to 7E and 8A to 8E, frequency characteristics of the first and second filter circuits 7 and 8 are also controlled by use of a controlling signal (phase comparison output) for controlling to be 90.degree. the phases of an input signal and an output signal of the secondary low-pass filter constituted by the reference filters 18 and 19. The low-pass filter 21 is for converting the error signal to a direct current, while the controlling circuit 22 is for temperature compensation. Numerals 24 and 26 are a capacitor and a resistor which are externally attached to terminals 23 and 25, respectively.
In the above-described conventional automatic frequency adjusting circuit, however, since the 1.3 MHz and 1.7 MHz filter circuits are adjusted by an adjusting signal generated by use of a 3.58 MHz signal, a 2 MHz frequency difference originally exists between an adjusting signal generating system and an adjusted system. As a result, it is impossible to provide precise adjustment. Since in a VCR, a 15.75 kHz horizontal synchronizing signal is present in addition to the 3.58 MHz color subcarrier, it is possible to generate an adjusting signal by use of the horizontal synchronizing signal instead of the color subcarrier. However, since the frequency difference from the adjusted system is also large in that case, the same problem occurs.
That it is impossible to provide precise adjustment means that the overall characteristic of the filter circuit 7 or 8 is automatically adjusted to be not a proper characteristic 50 but a characteristic 51 shown in FIG. 3. When adjustment is made in such a manner, since the frequency relationship of the input FM audio signal originally corresponds to the characteristic 50, if trap portions b0 and d0 which have frequencies to attenuate input signals are shifted to portions b and d, respectively, input signals which are present at the portions b and d are mostly attenuated. As a result, the signal-to-noise (S/N) ratio largely deteriorates. Moreover, the conventional automatic frequency adjusting circuit has drawbacks since its circuit scale is quite large.